diff --git a/src/Key/AbstractKey.php b/src/Key/AbstractKey.php
index dfa890d..7a4a59f 100644
--- a/src/Key/AbstractKey.php
+++ b/src/Key/AbstractKey.php
@@ -165,7 +165,7 @@ public function jsonSerialize(): array
     /**
      * @since 1.0.0
      */
-    public static function createFromJSON(string $json, KeyInterface $prototype = null): KeyInterface
+    public static function createFromJSON(string $json, ?KeyInterface $prototype = null): KeyInterface
     {
         $assoc = \json_decode($json, true);
 
diff --git a/src/Key/Rsa.php b/src/Key/Rsa.php
index 89c9ac2..48adea0 100644
--- a/src/Key/Rsa.php
+++ b/src/Key/Rsa.php
@@ -100,7 +100,7 @@ public function jsonSerialize(): array
      *
      * @return self
      */
-    public static function createFromJSON(string $json, KeyInterface $prototype = null): KeyInterface
+    public static function createFromJSON(string $json, ?KeyInterface $prototype = null): KeyInterface
     {
         if (!$prototype instanceof self) {
             $prototype = new static();